Passing string properly to eval() in python

Question

my question is in regard to eval() 'ing a string that contains trusted-user input. I am not sure how to properly package the string (first line after try:). An exception is raised on eval() in the example below. Any help would be highly appreciated. Here is an example code:

import ast

def do_set_operation_on_testids_file(str_filename1, str_filename2, str_set_operation):
  resulting_set = None
    with open(str_filename1) as fid1:
      with open(str_filename2) as fid2:
        list1 = fid1.readlines()
        list2 = fid2.readlines()

        try:
            str_complete_command = "set(list1)." + str_set_operation + "(set(list2))"
            resulting_set = ast.literal_eval(str_complete_command)
            for each_line in resulting_set:
                print(each_line.strip().strip('\n'))
        except:
            print('Invalid set operation provided: ' + str_set_operation)

Thanks very much!

What is the value of str_complete_command before you try to eval it? — Andy
– Andy ♦, Commented Jul 19, 2014 at 3:45
An exception is raised. But I am sure yet how to catch and print out the exception. — Mark Bronstein
– Mark Bronstein, Commented Jul 19, 2014 at 3:46
Instead of except, use except Exception, e and then include str(e) in your print statement. — TheSoundDefense
– TheSoundDefense, Commented Jul 19, 2014 at 3:48

alecxe · Accepted Answer · 2014-07-19 03:54:34Z

2

You don't need to use literal_eval() or eval() at all.

Use getattr() to get the set operation method by string:

>>> list1 = [1,2,3,2,4]
>>> list2 = [2,4,5,4]
>>> str_set_operation = "intersection"
>>> set1 = set(list1)
>>> set2 = set(list2)
>>> getattr(set1, str_set_operation)(set2)
set([2, 4])

Alternatively, you can pass an operator function instead of a string with set method name. Example:

>>> import operator
>>> def do_set_operation_on_sets(set1, set2, f):
...     return f(set1, set2)
... 
>>> do_set_operation_on_sets(set1, set2, operator.and_)
set([2, 4])

where and_ would call set1 & set2, which is an intersection of sets.

edited Jul 19, 2014 at 3:54

answered Jul 19, 2014 at 3:49

alecxe

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